A capacitive microphone typically comprises a membrane and a back plate, wherein an air gap is provided between the membrane and the back plate. Sound pressure waves applied onto the membrane force the membrane to vibrate due to a pressure difference over the membrane. In order to obtain a good omni-directional performance, the back side of the membrane is often acoustically isolated. Further, the membrane is usually connected to an acoustically closed back chamber influencing the membrane compliance and defining a lower cut of frequency. A tiny hole in the back chamber is required to compensate for slow changes in atmospheric pressure.
A modulation of the air gap between the membrane and the back plate due to sound pressure waves applied on the membrane result in an electrically detectable signal when using conductive materials for the membrane and the back plate. In this way, the membrane and the back plate, both provided with conductive surfaces, form a capacitor which capacity changes in relation to sound pressure waves applied to the membrane. Ideally, the back plate is a stiff plate and only the membrane is displaced by the sound pressure waves.
Such capacitive microphones known from the prior art often contain a membrane and a back plate that are made in a silicon Micro-Electro-Mechanical System (MEMS) process, while the back chamber is defined by the overall package or the capacitive microphone itself. So-called MEMS microphones are preferably used for mobile phones by integrating electronics with microphones into system in package (SiP) solutions, as conventional electret microphones do not have the desired form factor. The electronics in the microphone may comprise pre-amplifiers, biasing circuits, A/D converters, and signal processing and bus drivers.
The sensitivity of the microphone is determined by the compliance of the membrane, i.e. the flexibility of the membrane. The compliance is controlled by either the mechanical construction or the material parameters (after-fabrication stress and/or Young's modulus), wherein, depending on the design of the microphone, the mechanical construction or the material parameters dominate the performance.
An important performance parameter of such a microphone is the sensitivity to structural born sound, which is governed by undesired relative movement between the membrane and the back plate due to mechanical vibrations acting on the microphone as a whole. The so-called body noise is a disturbing effect of the microphone. One example of such disturbing effect is cross talk of a mobile phone's own speaker into the microphone which has a non-linear transfer function. Such disturbing effects cannot be compensated for by signal processing.
Another problem that most mobile phone users have to deal with is the need to suppress as quickly as possible the ring tone of a phone because they have received a call at a moment that the ring tone sound is highly undesirable (e.g. when in company of others, during a show or other type of performance, during presentations and conferences, in libraries, courts, etc). The user needs to find the phone, retrieve it from his or her pocket or bag, and find the right button (sometimes in the dark or as inconspicuously as possible). This takes time and can irritate the surrounding people.
Accordingly, it is the object of the invention to provide a microphone, which is less sensitive to mechanical vibrations and to provide a method for manufacturing a microphone that provides body noise cancellation.
This object is addressed by a method for manufacturing a micromachined microphone and accelerometer from a wafer having a first layer, the method comprising the steps of dividing the first layer into a microphone layer and into an accelerometer layer, covering a front side of the microphone layer and a front side of the accelerometer layer with a continuous second layer, covering the second layer with a third layer, forming a plurality of trenches in the third layer, removing a part of the wafer below a back side of the microphone layer, forming at least two wafer trenches in the wafer below a back side of the accelerometer layer, and removing a part of the second layer through the plurality of trenches formed in the third layer.
Accordingly, it is an essential idea of the invention to provide an accelerometer, which is preferably provided as a one-dimensional accelerometer, in close vicinity to the microphone, wherein the accelerometer is processed in the same die with the microphone. Such a microphone and accelerometer according to the invention allow for suppressing mechanical vibrations, leading to an improved signal to noise ratio. The accelerometer may facilitate further functionality when used in a cell phone, such as ending a conversation by shaking the cell phone, enabling silent mode when placing the cell phone at its back or front.
Preferably, the microphone is provided as a MEMS capacitive microphone. It is a further advantage of the invention that such an accelerometer can be produced by the same process flow as required for a capacitive MEMS microphone, without changing the physical size of the MEMS microphone die. Doing so, local process variations, which may influence critical parameters for sensitivity, such as stress of a layer, are optimized and no additional masks are necessary for the realisation of the accompanying one-dimensional accelerometer. It is further preferred that the first layer is provided as silicon layer, the second layer is provided as an oxide layer and the third layer is provided as a polysilicon layer.
The micro machined microphone and accelerometer according to the invention can be manufactured using techniques known from the prior art, such as etching using a reactive ion edge, deep reactive ion edging (DRIE), or alternatively wet anisotropic edging in potassium hydroxide (KOH) or tetramethylammonium hydroxide (TMAH), or sacrificial layer edging. Preferably, the second layer is provided as tetra alcoxysilan (TEOS).
According to another preferred embodiment of the invention, the trench in the third layer covering the intersection between the microphone layer and the accelerometer layer via the second layer is wider than the trenches in the third layer covering the microphone layer and/or the accelerometer layer via the second layer. This means that the microphone layer, which preferably forms a microphone membrane, is covered by the third layer, comprising a plurality of trenches, and that the accelerometer layer, which preferably forms an accelerometer membrane, is covered by the third layer comprising a plurality of trenches, and wherein the third layer is preferably removed in the area relatively covering the intersection between the microphone layer and the accelerometer layer.
According to another preferred embodiment of the invention, the at least two wafer trenches in the wafer below the back side of the accelerometer layer form an accelerometer wafer mass, which is defined by the outmost trench on the one side of the wafer below the back side of the accelerometer layer and by the outmost trench on the other side of the wafer below the back side of the accelerometer layer.
According to another preferred embodiment of the invention, the product of the mass of the microphone layer with the compliance of the microphone layer equals the product of the mass of the accelerometer layer and the accelerometer wafer mass with the compliance of the accelerometer layer. In other words, the microphone layer and the accelerometer layer including the accelerometer wafer mass are preferably designed in such a manner that they provide a co-phased response of equal amplitude for mechanical vibrations. This means further, that the accelerometer layer forming the accelerometer is preferably more sensitive to accelerations and less sensitive to sound pressure waves, due to the additional accelerometer wafer mass. It should be noted that the compliance is defined as the reciprocal of the stiffness of the membrane, i.e. the reciprocal of the stiffness of the microphone layer or the reciprocal of the stiffness of the accelerometer layer, respectively.
According to another preferred embodiment of the invention, the first layer is the device layer of a silicon-on-insulator (SOI) wafer. As known in the prior art, a SOI wafer preferably includes a top silicon layer, usually called the device layer, an intermediate insulator (oxide) layer, and a bottom silicon layer that is typically much thicker than the top silicon layer (approximately 650 microns). Alternatively, the first silicon layer can be provided as a silicon wafer.
The object of the invention is further addressed by an apparatus comprising a wafer having a first layer, wherein the first layer is divided into a microphone layer and into an accelerometer layer, and wherein a part of the wafer below a back side of the microphone layer is removed and at least two wafer trenches are formed in the wafer below a back side of the accelerometer layer, and a front side of the microphone layer and a front side of the accelerometer layer are covered with a continuous second layer, wherein the continuous second layer is covered with a third layer, wherein a plurality of trenches are formed in the third layer and wherein a part of the second layer is removed through the plurality of trenches formed in the third layer. This is advantageous over the prior art as the apparatus according to the invention allows for body noise cancellation in order to minimize structure borne sound.
In other words, such an apparatus according to the invention comprises a microphone, provided by the microphone layer forming a membrane and by the trenched second layer forming a back plate, and an accelerator provided by the accelerometer layer forming an accelerometer membrane and by the trenched continuous second layer forming a back plate. It is preferred that the first layer is provided as silicon layer, the second layer is provided as an oxide layer and the third layer is provided as a polysilicon layer.
According to another preferred embodiment of the invention, the microphone layer of the apparatus is adapted for generating a first electrical signal, wherein the first electrical signal is proportional to pressure applied to the microphone layer and/or to the accelerometer layer, and wherein the accelerometer layer is adapted for generating a second electrical signal, wherein the second electrical signal is proportional to pressure applied to the microphone layer and/or to the accelerometer layer. Preferably, the first electrical signal is generated due to a modulation of an air gap between the microphone layer and the trenched third layer forming a first capacitor and the second electrical signal is generated due to a modulation of the air gap between accelerometer layer and the trenched third layer forming a second conductor.
According to another preferred embodiment of the invention, the apparatus comprises a subtraction module which is adapted for subtracting the second signal from the first signal. This is advantageous, as the second electrical signal generated by the second conductor formed by the microphone layer representing structure borne sound due to undesired mechanical vibrations is subtracted from the first electrical signal generated by the first conductor of the microphone layer represent as a result of the subtraction an acoustic signal free of or nearly free of structure borne sound.
According to another preferred embodiment of the invention, the trench in the third layer covering the intersection between the microphone layer and the accelerometer layer via the second layer is wider than the trenches in the third layer covering the microphone layer and/or the accelerometer layer via the second layer. It is further preferred, and according to another preferred embodiment of the invention, that the at least two wafer trenches in the wafer below the back side of the accelerometer layer form an accelerometer wafer mass, which is defined by the outmost trench on the one side of the wafer below the back side of the accelerometer layer and by the outmost trench on the other side of the wafer below the back side of the accelerometer layer. According to another preferred embodiment of the invention, the product of the mass of the microphone layer with the compliance of the microphone layer equals the product of the mass of the of the accelerometer layer and the accelerometer wafer mass with the compliance of the accelerometer layer. It is further preferred that the first layer is a device layer of a SOI wafer.
The object of the invention is further addressed by a method of use of a micromachined microphone and accelerometer according to the invention for detecting a first electrical signal between the microphone layer and the third layer relatively covering the microphone layer, wherein the first electrical signal is proportional to pressure applied to the microphone layer and/or to the accelerometer layer, detecting a second electrical signal between the accelerometer layer and the third layer relatively covering the accelerometer layer, wherein the second electrical signal is proportional to pressure applied to the microphone layer and/or to the accelerometer layer, and subtracting the second electrical signal from the first electrical signal. This is advantageous over prior art, as the signal subtraction removes structure borne sound due to undesired mechanical vibrations in the acoustic signal.
The present invention also includes a method of use of an apparatus having a micro machined microphone and accelerometer:
detecting at least one jolt applied to the telephone, and
executing one action comprising any of silencing the phone, suppressing a ring tone, suppressing sound only, setting phone in quiet mode, answering call automatically.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.